Question: The numbers 1, 3, 6, 10, $\ldots$, are called triangular numbers, as shown geometrically here.  What is the $20^{\text{th}}$ triangular number?

[asy]

dot((0,0));
label("1",(0,-1.5));

dot((3,0));
dot((4,0));
dot((3,1));
label("3",(3.5,-1.5));

dot((7,0));
dot((8,0));
dot((9,0));
dot((7,1));
dot((7,2));
dot((8,1));
label("6",(8,-1.5));

dot((12,0));
dot((13,0));
dot((14,0));
dot((15,0));
dot((12,1));
dot((13,1));
dot((14,1));
dot((12,2));
dot((13,2));
dot((12,3));
label("10",(13.5,-1.5));

[/asy]
Solution: The 20th triangular number is $1 + 2 + 3 + \cdots + 20 = \frac{(20)(21)}{2} = \boxed{210}$.